Authored by: Stephen J. Mraz, Senior Editor [email protected]
Juno will become one of the biggest planetary spacecraft ever launched when it takes off next year on its five-year journey to Jupiter.
Jupiter, the largest planet in the solar system and visible with the naked eye from Earth, has been watched by astronomers since ancient times. And, beginning in 1973, the U. S. has sent seven unmanned probes to explore the planet. But its perpetual cloud cover and helium/hydrogen atmosphere have shrouded many basic facts about the planet. For example, scientists are still unsure how large the core is and what it is made up of, or if the planet even has a solid surface.
To learn more about the gas giant, NASA is preparing Juno to take off next August from Cape Canaveral onboard an Atlas V-551 rocket. The $700 million project should last six years — five years of travel time and one year orbiting Jupiter. But both getting there and surviving long enough to carry out its explorations presents a few challenges to aerospace engineers designing and building Juno.
Pulling power from the Sun
NASA has decided to power Juno strictly from solar cells. It could be because NASA’s usual power source for satellites, radioisotope thermal generators (RTG), are in short supply and needed for other missions. Or the organization has become more sensitive to concerns surrounding the risks of blasting nuclear material into space. NASA declines to elaborate.
Either way, the reliance on solar cells for a mission so far from the Sun is a first. That’s because Jupiter’s orbit is five times further from the Sun than Earth’s, so Juno will only receive 4% of the sunlight a satellite orbiting Earth would get. Fortunately, electrical engineers have made several advances in solar cells, making Juno’s cells 50% more efficient and radiation tolerant than silicon cells engineered for space missions two decades ago.
The spacecraft will carry three solar panels, each of which folds up into hinged segments for launch. Once deployed, they will provide 650 ft2 of solar cells, enough to generate at least 486 W when it first arrives at Jupiter. This will shrink to 420 W at the end of its one-year mission as radiation prematurely ages and degrades the cells. For comparison, if Juno simply orbited Earth, its panels would turn out about 15 kW.
Engineers also had to ensure Juno and its solar cells would be in the sunlight as much possible during its six-year mission. And thanks to a highly elliptical orbit around Jupiter, the satellite will remain in sunlight from launch until the end of the mission, (except for a 10-min stretch during an Earth fly-by as it accelerates on its way to Jupiter). Engineers also made sure the instruments onboard Juno would only need full power for about 6 hr during each of its 32 orbits of Jupiter. (Each orbit will last 11 days.)
Jupiter puts out more radiation than any other planet in the solar system — only the Sun throws off more damaging radiation. So NASA engineers had to take precautions to protect sensitive electronics.
The first line of defense is the path Juno will travel. Jupiter’s radiation belts form a huge doughnut that circles the planet’s equatorial region and extends 400,000 miles into space. So NASA engineers made sure Juno would initially approach Jupiter from above one of its poles. The spacecraft then establishes a highly eccentric orbit that skims 3,000 miles above Jupiter’s clouds, beneath most of the ionizing radiation. The spacecraft passes over the pole and continues on an elliptical trajectory until it is about 1.7 million miles away from Jupiter, then it curves back toward the planet. This eccentric orbit avoids most of the radiation belt and gives Juno a close-up look at Jupiter during part of the orbit. But the final four or five orbits do have Juno traveling through significant portions of the belt.
A titanium vault
Even with eccentric orbits, scientists estimate Juno will have to endure the radiation equivalent of 100 million dental X-rays during its mission near Jupiter. So the second line of defense is a six-sided titanium strongbox that surrounds Juno’s central electronics. This includes its command and data-handling equipment, power and data-distribution network, and about 20 other electronic assemblies. Engineers chose titanium rather than lead because a vault made of lead would be too soft to withstand the vibrations on take-off.
Each wall of the vault measures about 0.33-in. thick, covers nearly 9 ft2, and weighs 40 lb.
The vault, a first for NASA, won’t stop every electron, proton, or ion from hitting and damaging the equipment, but it should slow the aging effect radiation has on electronics. “Without this protective shield or radiation vault, Juno’s brain would get fried on its very first pass near Jupiter,” says Scott Bolton, Juno’s principal investigator based at the Southwest Research Institute in San Antonio.
To backup the vault, some electronic assemblies are constructed with tantalum or tungsten enclosures, two radiation-resistant metals. And some of the assemblies have their own separate metal vaults for even more protection. And to get the most bang for the buck, engineers arranged the electronic boxes and assemblies so that they shield one another.
The Juno mission is being managed by NASA’s Jet Propulsion Lab in Pasadena. The spacecraft itself is being built by Lockheed Martin in its Denver facility. And the Italian Space Agency is contributing several scientific instruments that will be carried onboard.
Juno will be the second spacecraft designed under NASA’s New Frontiers Program. The first, the Pluto New Horizon mission, was launched in January of 2006 and is scheduled to reach Charon, a moon of Pluto, in 2015. The program is set up to explore top-priority targets in the solar system with spacecraft and missions costing about $700 million or less.
When Juno reaches Jupiter, it will unleash its arsenal of nine scientific instruments and 25 sensors to explore the planet’s interior, atmosphere, and the magnetic fields and auroras that surround it. It will also measure the ammonia clouds in the atmosphere and the amount of water on the huge gas giant. Here are the instruments it will use:
Energetic Particle Detector: This instrument uses time-of-flight measurements rather than energy readings to calculate the energy and angular distribution of hydrogen, helium, oxygen, sulfur, and other ions in the polar magnetosphere.
Gravity Science Experiment: The goal of this experiment is to map the internal structure of the planet by measuring its complex gravity field. It relies on radio telemetry that sends data back to Earth to measure the precise location of Juno relative to Jupiter The spacecraft will rely on three low-gain antennae to transmit complete tracking data despite any changes in spacecraft orientation.
Jovian Aurora Distribution Experiment: This device will measure the energy and compositional distribution of particles in the polar magnetosphere. This data will help resolve questions about the plasma structure of the planet’s aurora.
JunoCam: A camera will take three-color images of Jupiter during Juno’s first seven orbits of the planet, giving scientists their first look good look at the planet’s poles. Image data will be processed and studied by colleges students as part of the Juno Education and Public Outreach effort. Images will have approximately 9.3 miles/pixel resolution. And after the seven-orbit design life, JunoCam will continue to operate as long as possible in the harsh radiation environment.
Jupiter Infrared Auroral Mapper: This Italian designed and built device includes a high-contrast IR imager and a spectrometer. It will probe the upper layer of the Jovian atmosphere and gather data on the chemistry of the auroral region and the magnetic field surrounding the planet. Readings from the IR imager should help scientists determine the amount of water and other substances in the atmosphere.
Magnetic Field Investigation: Two instruments, a fluxgate magnetometer and an advanced stellar compass, will let engineers map the planet’s magnetic field, determine the structure of the interior, and plot the three-dimensional structure of the polar magnetosphere. The compass will generate accurate data on where the spacecraft is pointed, while the magnetometer measures magnetic fields.
Microwave Radiometer: An array of six separate radiometers will probe the Jovian atmosphere to measure the amount of water and ammonia in it. The radiometer will use three antennae to receive signals at six different wavelengths.
Plasma Waves Instrument: It will measure the radio and plasma spectra in auroral regions, which should let researchers identify auroral currents that define Jupiter’s radio emissions.
UV Spectrograph: The second spectrograph onboard Juno, it records the wavelength, position, and arrival time of UV photons when the device’s slit aligns with Jupiter as Juno rotates. It is designed to provide scientists with spectral images of the UV auroral emission in the polar magnetosphere.